354 



COLLEGE ZOOLOGY 



Brain 



The brain (Fig. 234) has two large olfac- 

 tory lobes which are fused together, two 

 large cerebral hemispheres, diencephalon, 

 two large optic lobes, a very small cerebel- 

 lum, and a medulla (medulla oblongata), 

 which is produced by the broadening of the 

 spinal cord. 



The functions of the different parts of the 

 frog's brain have been partially determined 

 by experiments in which the parts were re- 

 moved and the effects upon the animals 

 observed. There is evidence that the cerebral 

 hemispheres are involved in associate mem- 

 ory, but the frog has a very low "l.Q." The 

 cerebrum is the seat of intelligence and vol- 

 untary control in higher animals. When the 

 diencephalon is removed with the cerebral 

 hemispheres, the frog loses the power of 

 spontaneous movement. When the optic 

 lobes are removed, the spinal cord becomes 

 more irritable; this shows that these lobes 

 have an inhibiting influence on the reflex 

 activity of the spinal cord. In man the 

 cerebellum is a center of coordination, but 

 experiments on the frog cerebellum have 

 produced conflicting results. Many activities 

 are still possible when everything but the 

 medulla is removed. The animal breathes 

 normally, snaps at and swallows food, leaps 

 and swims regularly, and is able to right 

 itself when thrown on its back. Extirpation 

 of the posterior region of the medulla re- 

 sults in early death of the frog. The brain 

 as a whole controls the actions effected by 

 the nerve centers of the spinal cord. "The 

 higher centers of the brain are comparable 

 to the captain of a steamer, who issues or- 

 ders to the man running the engine, when 

 to start and when to stop, and who has his 

 hand on the wheel so as to guide the course 

 of the vessel" (Holmes). 



Spinal cord 



The spinal cord (Fig. 233) extends back- 

 ward from the medulla and ends in the 

 urostyle. It is surrounded by two mem- 

 branes, an outer dura mater and an inner 

 pia mater. The cord is composed of a cen- 



tral mass of gray matter (Fig. 235) consist- 

 ing mainly of nerve cells, and an outer mass 

 of white matter made up chiefly of nerve 

 fibers. A median fissure occurs both in the 

 dorsal and in the ventral side of the cord, 

 and a central canal lies in the gray matter 

 and communicates anteriorly with the cavi- 

 ties of the brain. 



Spinal nerves 



The relation of the spinal nerves to the 

 spinal cord and the paths taken by nervous 

 impulses are indicated in Fig. 235. There are 

 10 pairs of spinal nerves in the frog (Fig. 

 233). Each arises by a dorsal and a ventral 

 root, which arise from the gray matter of 

 the cord. The two roots unite to form a 

 trunk, which passes out between the arches 

 of adjacent vertebrae. The two largest 

 nerves are ( 1 ) the brachials, each of which 

 is composed of the second pair of spinal 

 nerves and branches from the first and third 

 pairs— these are distributed to the forelimbs 

 and shoulders; and (2) the sciatics, which 

 arise from plexuses composed of the seventh, 

 eighth, and ninth spinal nerves, and are dis- 

 tributed to the hindlimbs. 



Autonomic system 



This system (Fig. 233) consists of two 

 principal trunks, which begin at the cranium 

 and extend posteriorly, one on each side of 

 the vertebral column. Each trunk is pro- 

 vided with 10 ganglionic enlargements at 

 the points where branches from the spinal 

 ner\'es unite with it. The nerves of the auto- 

 nomic system are distributed to internal 

 organs and regulate many functions that are 

 not under the control of conscious or volun- 

 tary action, such as heart beat, secretions, 

 movements of the digestive tract, and res- 

 piratory, urogenital, reproductive systems, 

 and others. 



Sense organs 



The principal sense organs are the eyes, 

 ears, and olfactory organs. There are many 

 smaller structures on the surface of the 



